1. Field of the Invention
The present invention relates in general to relays and in particular to a relay having a pulse-width modulated control voltage.
2. Description of Related Art
FIG. 1 is a simplified sectional elevation view of a typical prior art normally open relay 10 including a glass tube 12 containing a pair of conductive reeds 14, 15 serving as the relay""s contacts 16 for selectively providing a signal path between two circuit nodes. A wire 17 wrapped many turns around tube 12 forms a coil 18. Reeds 14, 15 are normally spaced apart, but when a current passes through coil 18, coil 18 produces magnetic flux causing reed 14 to contact reed 15 so that a current may flow through the relay contacts 16.
FIG. 2 is a schematic diagram illustrating a typical relay control system 25 for driving relay 10. Control system 25 includes a switching power supply 28 that can produce a DC signal VS of a desired voltage from an AC signal. As illustrated in FIG. 2, power supply 28 includes a rectifier 28A and capacitor 28B for converting the AC signal to a high voltage DC signal. A switch 28C controlled by a pulse width modulation circuit (PWM) 28D couples the DC signal across the primary winding of a transformer 28E. The transformer""s secondary winding is connected across a diode 28F and a capacitor 28G. The power supply""s DC output voltage VS developed across capacitor 28G is a function of the duty cycle with which pulse PWM circuit 28D closes switch 28C. PWM circuit 28D monitors VS and adjusts the duty cycle to keep VS at a desired level.
Relay control system 25 also includes a switch 26 linking a power supply 28 to a circuit node 23. A limiting resistor 24 links node 23 to a node 27. Relay coil 18 is connected between node 27 and ground. (Limiting resistor 24 may be a separate discrete component as shown in FIG. 2 or may be implemented as the resistance of the wires forming coil 18.) Relay 10 includes a diode 29 connected across coil 18. When signal SW is applied, switch 26 closes, a current I begins to flow in coil 18 and power supply 28 begins to supply energy to coil 18 and resistor 24 at a constant rate. Initially, current I is very low and resistor 24 dissipates little of the energy output of power supply 28. Coil 18 stores most of the energy output of power supply 28 in a magnetic field. However the current is proportional to the strength of the coil""s magnetic field and the strength of the coil""s magnetic field is proportional to the amount of energy stored in the field. Hence the coil current increases and the magnetic field becomes stronger as coil 18 continues to receive energy from supply 28 and store it in the magnetic field.
The rate at which resistor 24 dissipates energy supplied by power supply 28 is proportional to I2R, where R is the resistor""s resistance. Little current passes through resistor 24 when switch 26 initially closes and therefore coil 18 stores little energy in its magnetic field. Therefore resistor 24 initially dissipates little of the energy produced by supply 28. However as coil 18 begins to store increasing amounts of energy in its magnetic field, it permits an increasing amount of current I to flow through resistor 24, and the resistor begins to dissipate an increasing proportion of the output energy of power supply 28. Coil 18 stops adding energy to the magnetic field when coil current I reaches a substantially constant steady state level Iss after all initial transients or fluctuating conditions have settled. At that point resistor 24 will dissipate all of the energy being produced by supply 28. The resistance of resistor 24 (in combination with the inherent resistances of coil 18, switch 26 and supply 28) limits the steady state level Iss at which the coil current I levels off after switch 26 closes. Resistor 24 therefore acts as a xe2x80x9ccurrent limitingxe2x80x9d resistor inserted into the circuit to limit the flow of current, thereby preventing excessive current from damaging other parts of the circuit.
When switch 26 opens, the magnetic field surrounding coil 18 continues to induce a current I within coil 18 and there is no instantaneous change in its amplitude. However rather than circulating in the loop including coil 18, supply 28, switch 26 and resistor 24, coil current I instead circulates in the loop including coil 18 and the diode 29 connected across the coil""s terminals. As the inherent resistances of diode 29 and coil 18 dissipate the energy stored in the magnetic field, the field collapses and the coil current amplitude tapers off to zero.
The intensity of the magnetic field coil 18 produces is proportional to the product of the amplitude of the current passing through coil 18 and the number of turns of the coil about tube 12. A typical relay coil 18 will include a large number of turns to minimize the amount of steady state current Iss needed to operate relay 10 because this also minimizes the power resistor 24 dissipates. The power P that resistor 24 dissipates in response to the steady state current is as follows:
P=Iss2Rxe2x80x83xe2x80x83[1]
By doubling the number of coil turns we can reduce the required steady state current Iss while still maintaining the same steady state field intensity. To reduce Iss, we double R. While equation [1] tells us doubling R increases power dissipation by a factor of two, it also tells us that reducing Iss in half decreases power dissipation by a factor of four. Thus, the net effect of doubling the number of coil turns and doubling the size of resistor 24 is to cut the resistor""s power dissipation in half. Hence to reduce power consumption, relay coils typically have many turns. However as we add turns to a relay coil, we not only add to the cost of making the relay, we also add to the relay""s physical size. A relay""s coil typically contributes more than half of its thickness.
Thick relays can be problematic in applications where large numbers of relays must be packed into a small volume. For example, an integrated circuit (IC) tester typically uses relays to route signals between test circuits and terminals of an IC device under test (DUT). It is helpful to position the test circuits as closely as possible to the DUT terminals so that signal paths between the test circuits and the DUT""s input/output terminals are very short. Since a large number of relays must reside between the test circuits and the DUT, we want to be able to pack as many relays as possible into a small space. However since relays having many turns are thick, large numbers of them cannot be packed into a small space.
We could use thin relays having fewer coil turns, but as discussed above, conventional controllers for such relays would generate substantial amounts of heat in their current limiting resistance. Therefore what is needed is a relay controller that can drive a high current relay having relatively few coil turns without incurring substantial power loss in current limiting resistance.
The present invention is directed to a relay controller using pulse-width modulation to control the current in a relay coil. Rather than continuously linking a power supply to a relay coil and using resistance to limit coil current when a control signal indicates the relay coil is to produce the magnetic field, a relay control system in accordance with the invention intermittently connects the power supply to the coil with a controlled frequency and duty cycle. No current limiting resistor is required because the frequency and duty cycle with which the power supply is connected to the coil limits the steady-state amplitude of the current passing through the coil.
In some embodiments of the invention the frequency and duty cycle with which the power supply is connected to the relay coil is fixed to a level that keeps the coil current within a steady state range for which the magnetic field intensity is sufficient to operate the relay coils. In other embodiments of the invention, the controller monitors the coil current and adjusts the duty cycle with which the power supply is connected to the relay to keep the coil current in the appropriate range.
Since a relay controller in accordance with the invention does not rely on current limiting resistance in series with the relay coil, the relay controller dissipates energy at a relatively low rate, even when the coil has very few turns and requires a high steady state current to produce a magnetic field of sufficient intensity. Having relatively few turns in its coil, a relay driven by a relay control system in accordance with the invention can be compact and inexpensive to fabricate.